As an emission type electronic displaying device, there is an electroluminescence device (ELD). As materials constituting the ELD, there is an inorganic electroluminescent element or an organic electroluminescent element. The inorganic electroluminescent element has been used for a plane-shaped light source, but a high voltage alternating current has been required to drive the element. An organic electroluminescent element has a structure in which a light emission layer containing a light emission compound is arranged between a cathode and an anode, and an electron and a positive hole were injected into the light emission layer and recombined to form an exciton. The element emits light, utilizing light (fluorescent light or phosphorescent light) generated by deactivation of the exciton, and the element can emit light by applying a relatively low voltage of from several to several decade volts. Further, the element has a wide viewing angle and a high visuality since the element is of self light emission type, and the element is a complete solid element, and the element is noted from the viewpoint of space saving and portability.
However, in the organic EL element for practical use, an organic EL element is required which efficiently emits light with high luminance at a lower power.
In U.S. Pat. No. 3,093,796, there is disclosed an element with long lifetime emitting light with high luminance in which stilbene derivatives, distyrylarylene derivatives or tristyrylarylene derivatives are doped with a slight amount of a fluorescent compound.
An element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a slight amount of a fluorescent compound (Japanese Patent O.P.I. Publication No. 63-264692), and an element is known which comprises an organic light emission layer containing an 8-hydroxyquinoline aluminum complex as a host compound doped with a quinacridone type dye (Japanese Patent O.P.I. Publication No. 3-255190).
When light emitted through excited singlet state is used, the upper limit of the external quantum efficiency (next) is considered to be at most 5%, as the generation ratio of singlet excited species to triplet excited species is 1:3, that is, the generation probability of excited species capable of emitting light is 25%, and further, external light emission efficiency is 20%. Since an organic EL element, employing phosphorescence through the excited triplet, was reported by Princeton University (M. A. Baldo et al., Nature, 403, 17, p. 151-154 (1998)), study on materials emitting phosphorescence at room temperature has been actively made. As the upper limit of the internal quantum efficiency of the excited triplet is 100%, the light emission efficiency of the excited triplet is theoretically four times that of the excited singlet. Accordingly, light emission employing the excited triplet exhibits the same performance as a cold cathode tube, and can be applied to illumination.
It is necessary that when a phosphorescent material is used as a dopant, the maximum wavelength of light which a host compound emits, be in the region shorter than the maximum wavelength of light which the phosphorescent material emits, but in addition, there exist other requisites to be satisfied.
Several proposals with respect to the phosphorescent material were made in “The 10th International Workshop On Inorganic and Organic Electroluminescence (EL '00, Hamamatsu). For example, Ikai et al. use, a hole transporting compound as a dopant of a phosphorescent material, M. E. Tompson et al. use, as a host compound of a phosphorescent material, various kinds of electron transporting compounds, which are doped with a new iridium complex, and Tsutsui et al. obtain high light emission efficiency due to introduction of a hole blocking layer.
The host compounds of phosphorescent compounds are disclosed in for example, C. Adachi et al., “Appl. Phys. Lett., 77, pp. 904 (2000)”, but an approach from a new aspect with respect to characteristics required in the host compounds is necessary to obtain an organic electroluminescent element emitting light with high luminance.